4 cytochrome c++ + O2 + 8H+(in) => 4 cytochrome c+++ + 2H2O + 4H+(out)
The proton stoichiometry for this enzyme is confusing because some protons are used to form water, and electrons from cytochrome c must cross the membrane to react with oxygen in the matrix space. Reduction of oxygen is a four electron process, but most substrates donate two electrons to the respiratory chain, and cytochrome c is a one electron carrier. Expressed on the usual common basis, this enzyme exports two protons and four positive charges per atom of oxygen reduced to water.
The complex contains ten protein subunits. The three largest subunits are coded by mitochondrial DNA and the remainder imported from the cytosol. There are two atoms of copper, referred to as CuA and CuB, and two chemically identical, but spectrally distinct molecules of heme a. These are described as heme a and heme a3.
The poisons cyanide, carbon monoxide and azide all inhibit this enzyme by binding to the catalytic centre.
It is important that cytochrome oxidase completes the reduction of oxygen to water without releasing the toxic intermediates superoxide or peroxide. Mitochondria contain a distinct manganese dependent form of superoxide dismutase (imported from the cytosol) which protects them against the toxic effects of accidentally generated superoxide. Mice completely lacking this gene die within a few days from dilated cardiomyopathy, and in humans gene targeting defects lead to premature ageing.
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